The production of hydrogen chloride during combustion of some coals, petroleum fractions and various wastes has challenged combustion plant operators and regulators over many years. The chlorides produced can be effectively controlled by processes including wet scrubbing, however the presence of soluble chloride can have adverse effects on calcium carbonate based scrubbers.
When soluble chlorides reach a wet scrubber, such as a single-loop, open-tower countercurrent limestone wet scrubber, the reactivity of the scrubbing slurry will be adversely affected because chlorine in any form, such as chlorides, tend to be highly soluble and destroy the equilibrium within the scrubber by freeing some of the calcium from its primary function of capturing the SO2 as CaSO3. As pointed out in U.S. Pat. No. 5,635,149 to Klingspor, et al., to maintain reactor efficiency, the chloride content should be monitored and sorbent slurry removed as necessary and replaced with fresh sorbent. This is costly from the raw material standpoint and can decrease the quality of the gypsum produced by reducing the residence time in the reaction tank where oxygen is supplied to convert the CaSO3 to gypsum, CaSO4, which can provide needed revenue if of sufficient quality.
If it were possible to control the amount of chloride entering a wet scrubber designed for sulfur oxides removal, the operation of such scrubbers could be improved by reducing frequency of removing slurry based on chloride concentration exceeding a set limit. If the frequency were decreased, other factors, such as gypsum quality, gypsum particle size, the capture of other contaminants, and the like, could be used for process control. Control of the chloride concentration entering the scrubber could increase reaction tank residence times and decrease blow down from the tank.
The composition of wet scrubber wastewater effluent streams (e.g. blow down, filtrate from gypsum filtration, etc.) are primarily composed of chloride salts (e.g. calcium chloride) and other dissolved solids, usually in the range of 5,000 to 40,000 mg/L, see Shaw, William A., Power Magazine, Oct. 1, 2011, pages 56-62. Discharge of wastewaters is increasingly being regulated owing to the presence of trace toxic elements in the water, including arsenic, selenium, and boron. Increasingly, power plants are being held to zero liquid discharge limits. Options for zero liquid discharge include capital-intensive wastewater treatment systems followed by evaporation towers for the crystallization of inorganic salts. Evaporation towers can have large parasitic load requirements, on the order of 18-35 kWh/metric ton of water. Significant capital reductions can be achieved by removal of chlorides prior to entering the scrubber thereby minimizing the total volume and frequency of blow down, which is typically controlled by the chloride concentration in the wet scrubber liquor.
The prior art has dealt with the problem of halide build up in scrubber tanks, but is still awaiting an effective and economical solution. Additives to the tank are costly and affect other chemistries. Prescrubbers create large amounts of waste water that must be treated or disposed of. Post-scrubber treatment of blow down must be either treated to remove regulated contaminants (e.g. boron, arsenic, selenium), or water must be evaporated leaving a solid waste to be landfilled.
A low capital alternative to HCl reduction ahead of wet scrubbers is sorbent injection. Sorbent injection can be used to reduce the chloride concentration entering the scrubber; however, existing sorbents (e.g. trona, sodium bicarbonate, and hydrated lime) react with both SO2 and HCl. Typically, flue gas SO2 concentrations (e.g. 1500-3000 ppmv) are several orders of magnitude greater than HCl concentrations (e.g. 1-200 ppmv). Owing to poor selectivity for HCl, sorbent injection rates can often exceed practical and economic limits.
Accordingly, there is a present need for a process that can reduce the amount of chloride entering a wet scrubber.